Yolande Barbaux

Oxidation of isobutane on a heteropolycompound hydrogen reservoir

Abstract The selective oxidation of isobutane to methacrylic acid (MAA) has been studied on Cs1.6H2.4P1.7Mo11V1.1O40 which is a heteropolycompound (HPC). The oxidation mechanism isobutane → isobutene → MAL → MAA has already been proposed previously with a rate determining step corresponding to the first step of the reaction, i.e., to dehydrogenation of isobutane leading to isobutene. Here we show that the HPC which is active in the oxidation of isobutane possesses, in the reduced state, anionic vacancies able to store reactive hydrogen species H ∗ presenting the ability to diffuse through the solid. Such a result is of great importance since dehydrogenation requires the abstraction of hydrogen from the molecule which could be performed by a lacunar phase. Therefore, a mechanism of the dehydrogenation step is proposed, involving a heterolytic abstraction of a H− species from the alkane.

Wacker oxidation of various olefins in the presence of per(2,6-di-O-methyl)-β-cyclodextrin: mechanistic investigations of a multistep catalysis in a solvent-free two-phase system

Oxidation of various olefins into methylketones by a catalytic system constituted of palladium, copper, phosphomolybdovanadic acid and per(2,6-di-O-methyl)-β-cyclodextrin is investigated. The role of each redox catalytic system component is discussed from experiments under argon atmosphere and from vanadium NMR spectroscopic studies. The influence of various parameters, such as the per(2,6-di-O-methyl)-β-cyclodextrin concentration and the nature of the phosphomolybdovanadic acid is also reported. Finally, a catalytic cycle is also proposed.

Phosphomolybdic acid (H3PMo12O40) as a catalyst for the vapour-phase oxidative dehydrogenation of isobutyric acid : kinetic parameters of supported and unsupported catalysts. Role of water.

Abstract In order to determine the catalytic properties of molybdophosphoric acid used recently in the oxydehydrogenation of isobutyric acid, a systematic kinetic study of the formation of three main products has been undertaken. For each of these three products, propene, acetone and methacrylic acid, the results are explained in terms of a steady-state mechanism of the oxidised sites. Related to the morphology of these compounds, we show that these three products result from three different specific sites. The selectivity of the catalyst can be explained by these results. The important role played by water on these compounds has not been studied in any previous work concerning the catalytic oxydehydrogenation of isobutyric acid. The adsorption equilibrium constants of water on each site have been determined from steady state kinetic readings. This confirms the presence of three types of different sites, and explains the dependence of activity and selectivity on the partial pressure of water.

Effect of potassium on the surface potential of titania

The surface potential, χ, of several commercial titania powders has been measured by the vibrating condenser method in the temperature range 100–450 °C in air. The K impurity detected by XPS on some of titanias lowers the χ value of TiO2 considerably. A series of K-doped TiO2 anatase samples containing from 1.1 to 11 K atoms nm–2 was prepared and examined by surface potential and XPS techniques. The surface potential of anatase decreases linearly with increasing potassium content up to ca. 2.4 K atoms nm–2, and then remains constant. The surface potential of K-doped samples varies to a smaller extent with temperature compared with undoped anatase. On the basis of the surface potential and XPS data obtained, it is suggested that the mode of K deposition on the surface of anatase changes at a coverage of about 2 K atoms nm–2.

Identification of sites active in oxidation of butene-1 to butadiene and CO2 on CO3O4 in terms of the crystallochemical model of solid surfaces

Abstract The aim of this work is to help clarify the localisation and performance of active sites on the surface of Co 3 O 4 , engaged in oxidation of butene-1 to butadiene and CO 2 . A number of quoted experimental studies (by Barbaux, Beaufils, Bonnelle et al. ) serves as an experimental basis. According to the bond-length—bond-energy concept (Ziolkowski, J. Solid State Chem ., 57 (1985) 269) and its application to solid surfaces (Ziolkowski, Surf. Sci ., 209 (1989) 536) the most probable surface structures are indicated. These are the B(111) slice containing 4 Co 2+ ions over 16 O 2− and the B(110) slice containing 4 Co 3+ + 8 O 2− over 4 Co 2+ + 2 Co 3+ + 8 O 2− per surface unit cell in both cases. A crystallochemical model of active sites (Ziolkowski, J. Catal ., 84 (1983) 317; ibid ., 100 (1986) 45) is used to demonstrate why butene-1 is selectively transformed to butadiene and CO 2 on B(111) and B(110), respectively. The main difference between the two faces considered consists in the lifetime of the active complex and the number and degree of undersaturation of active surface oxygens surrounding the site on which butene is expected to be adsorbed. A detailed geometrical scheme and an energy pathway of the reaction butene → butadiene are proposed and discussed.

Effect of the sequence of potassium introduction to V2O5/TiO2 catalysts on their physicochemical properties and catalytic performance in oxidative dehydrogenation of propane

Abstract Two series of K-promoted V 2 O 5 /TiO 2 catalysts were prepared by: (a) deposition of vanadia on K-doped TiO 2 support (TiK V preparations), and (b) deposition of K on vanadia-titania catalysts (TiV K preparation). They were characterized by 51 V NMR, XPS, surface potential (work function) techniques and isopropanol decomposition, a probe reaction for the acid-base properties, and tested in oxidative dehydrogenation of propane (ODH). It has been found that the sequence of the K introduction in the preparation step is preserved in the calcined preparations, with more potassium being present on the surface of TiV K than TiK V catalysts. The vanadium species on TiK V samples include V 2 O 5 and polymeric [VO x ] γ species. The TiV K sequence leads to the formation of potassium vanadates (KV 3 O 8 and possibly KVO 3 ), and to the decrease in the amount of V 2 O 5 . The TiV K catalysts are more active and selective in the ODH of propane, and more active in isopropanol dehydrogenation to the acetone (thus more basic) than the TiK V samples.

OXYDEHYDROGENATION OF PROPANE ON V2O5/TIO2 CATALYST : KINETIC AND MECHANISTIC ASPECTS

Catalytic oxydehydrogenation of propane and work function measurements in similar conditions have been performed on V2O5/TiO2 catalyst. The oxidation mechanism C3H8 [graphic omitted] C3H6 [graphic omitted] COx+ H2O has been kinetically modelled which permits theoretical evolution of the selectivity as a function of the conversion, for various ratios of K=k1/k2. A good correlation is obtained between the experimental and theoretical curves, validating the kinetic modelling. The value obtained for K is 1, so the reactivity of propene is much higher than that of propane and justifies the observed yield limitation. Moreover, the rate-determining step of propane ODH reaction as well as of propene oxidation is attributed to the attack of the oxidized site by the hydrocarbon. The value of K, which depends on the type of catalyst may be used to evalute the selectivity to propene and for classification of oxydehydrogenation catalysts. The work function measurements give the nature and charge of the various oxygen species (O2–, O–, O2–) involved in the reaction, and O2– is found to react with propane.

Observation of the V2O5(001) surface using ambient atomic force microscopy.

Constant force images of the V2O5(001) surface were recorded in ambient conditions with atomic force microscopy. All images exhibit the 11.5 A × 3.5 A. periodicity expected for a bulk terminated surface. However, images reveal differences from the ideal structure. The experimental results are interpreted in terms of preferential adsorption sites for water molecules. Because these sites are thought to influence the catalytic properties of the surface, their characterization is an important step towards understanding how the atomic-scale structure of a surface influences its properties.

Effect of potassium addition to the TiO2 support on the structure of V2O5/TiO2 and its catalytic properties in the oxidative dehydrogenation of propane

Vanadium oxide has been deposited by a grafting technique onto TiO2 anatase, both pure and doped with potassium [(1.2 and 2.5) atoms nm–2]. The V content varied between 0.1 and 20 atoms nm–2[0.01–2 V2O5 monolayers (ML)]. The prepared samples were characterized by X-ray photoelectron spectroscopy (XPS), 51V magic-angle spinning (MAS) NMR and a surface potential (SP) technique and tested as catalysts in the oxidative dehydrogenation (ODH) of propane and propan-2-ol decomposition, a probe reaction for acid–base properties. From the XPS and SP data it has been inferred that VOx are located beside the K centres on the bare surface of TiO2 with the lower K content sample, whereas they cover the K-doped fraction of the surface for the sample with higher K content. Monomeric and polymeric VOx species and V2O5 were detected by 51V NMR on pure and K-doped catalysts. For the K-doped samples the polymeric species were observed only at high V content and new tetrahedral VOx species and traces of KVO3 appeared. It has been found that the presence of K on the TiO2 surface leads to (a) a decrease in the reducibility of the vanadia phase at low V content; (b) a decrease in the surface potential (electronic work function); (c) a decrease in acidity and increase in basicity and (d) a decrease in the total activity for ODH of propane. The pattern of the activity and selectivity changes with the total V content depends on the amount of K on the support surface: with K 1 ML) are required to obtain the same catalytic performance. Polymeric [VOx] species seem to be more active and selective in the ODH of propane than monomeric species or bulk V2O5.

A very useful and efficient Wacker oxidation of higher α-olefins in the presence of per(2,6-di-O-methyl)-β-cyclodextrin

Abstract Oxidation of higher α-olefins (C 8 C 16 ) in a two phase system with multicomponents catalytic system, i.e. PdSO 4 /H 9 PV 6 Mo 6 O 40 /CuSO 4 and per(2,6-di- O -methyl)- β -cyclodextrin) gives the corresponding 2-ketones in high yields (>90 %).